FIELD OF THE INVENTION
The present invention relates to the field of radio frequency signal transmission and more specifically concerns a procedure and a circuit for adaptive compensation of the gain distortions of a microwave amplifier with linearizer.
As known, a power amplifier behaves linearly, i.e. has gain and phase characteristics which are constant with input signal power variation and generally only for a certain range of values of said power. In a generic amplifier, the deviations from linearity consist principally of a gain compression and an increase in the input/output phase shift for high input signal power levels. Said nonlinearities produce amplified signal distortion and hence an increase in intermodulation noise. Transmission of such a signal can cause serious inconvenience for the receivers, as for example a high value of BER in case of digitally modulated signals. To obviate this inconvenience there have been in use for some time appropriate linearized circuits made in accordance with different circuitry types, e.g. with predistorter, which mainly involve the field of application of the present invention. As is known, a predistorter comprises non-linear elements which predistort the amplitude and phase of a radio frequency signal with low power level depending on the power level of said signal and the magnitude of the distortions which the amplifier would introduce on the original signal. More specifically, the predistortion trends are pre-set in such a manner that they are equal and opposite to the distortion trends otherwise introduced on the amplifier signal.
A predistortion linearizer has the advantage of speedy operation, and is hence especially well suited upstream of the microwave signal amplifiers. On the other hand, in an amplifier linearized in this manner, there are still possible small gain and phase characteristic variations, due to changes in operating conditions. This is because the gain and phase predistortion trends are initially pre-set in the design phase and are not changed during amplifier operation, except for sporadic manual adjustments when the amplified signal is too degraded. Since said trends accurately reproduce the distortion trends evaluated for a specific amplifier work point, when the work point moves slightly because of the aforementioned thermal drift or ageing phenomena, the distortion trends change accordingly and hence no longer correspond exactly to the predistortion trend. On the other hand the predistorter does not have a feedback configuration, i.e. capable of making the appropriate corrections automatically. Of course this drawback is more sensitively apparent during input signal power peaks.
A second shortcoming of amplifiers linearized by predistorters arises from the fact that the gain of an amplifier has never perfect flat band, and therefore any selection of a new transmission channel, made in the amplifier pass band, necessarily involves a resetting of the gain thereof to adjust the new gain to the previous predistortion curve.
To remedy the above shortcomings there are known linearization procedures and circuits based on the fact that it is possible to acquire knowledge of when the amplifier begins to distort merely by observing the levels of the intermodulation products of a higher order at the amplifier output, since in this case they increase. Said known linearizers therefore include an appropriate feedback system comprising circuits for measurement of the power of the intermodulation products, and circuits using said measurement to generate a signal for correction of the predistortion characteristic in order to recover perfect amplifier linearity.
The main drawback of said linearizers is that there must be made a very accurate measurement of very low power levels such as that of the intermodulation products. In addition, said measurement must be made within well delimited frequency bands. It is clear that the feedback system must therefore include unavoidably costly and sophisticated means which increase the amplifier cost.
In a few cases, as in the European patent application EP-A-0435578, said linearization procedures and circuits are subjected to a simplification mainly consisting in measuring the average power of the input, or output, signal and in using the average value for correction of the predistortion characteristic, in order to recover perfect amplifier linearity. The main drawback of said simplified linearizers is that no precise information about distorsion is obtained out, so that the amount of linearization is insufficient; an acceptable grade of linearization should be carried out only by means of an increasing of the circuitry complexity.